To live, aerobic organisms metabolize oxygen to generate energy. During this process, cells create reactive oxygen species (ROS). ROS can attack all cellular constituents, including lipids, proteins, and DNA. Such oxidative damage has been associated with the aging process and human disease, namely cancer and neurodegeneration. We have worked to define the biochemical and cellular processes for repairing oxidative DNA damage. In particular, we have delineated the structure-function mechanisms and biological contributions of specific proteins that participate in the base excision repair (BER) pathway. This process involves the recognition and excision of DNA damage, and restoration of the native genetic material. Defects in DNA repair give rise to mutations or cell death, leading to the development of disease. Much of our effort has involved defining the biochemical functions of Ape1, the major human protein for repairing abasic sites in DNA, a frequent genetic damage. We have demonstrated that Ape1 contributes to the repair of 3?-modifications in DNA as well, such as mismatches, phosphate groups, phosphogycolates, and tyrosyl residues. More recent work has found that Ape1 cleaves at AP sites in single-stranded regions of complex, biologically-relevant DNA structures, such as bubble and fork intermediates. These findings expand the known repertoire of substrates processed by this enzyme, and suggest novel functions for Ape1 likely coupled to transcription and/or replication. We are currently exploring for potential cooperativity between Ape1 and CSB, a transcription-related repair protein found defective in the human premature aging disorder, Cockayne Syndrome. Last, our work has found that the environmental metal, lead, is a potent inhibitor of Ape1 activity, suggesting a means by which this heavy metal may elicit its co-carcinogenic effects. In addition to the investigations above, we have initiated studies to determine the biochemical and cellular contributions of XRCC1, a major single-strand break repair (SSBR) factor. This protein functions primarily as a scaffold component, orchestrating specific protein-protein interactions required for efficient DNA repair. Recent work has identified associations of XRCC1 with proteins defective in the human neurodegenerative disorders AOA1 (Aprataxin) and SCAN1 (TDP1). We reported a novel link between XRCC1 and DNA replication, as XRCC1 was found to directly interact and co-localize with the replication factor PCNA. Additional studies by our group argue against a role for XRCC1 in the early steps of BER, and indicate a biologically-relevant role for its interaction with DNA polymerase beta and in the subsequent stages of SSBR, namely DNA nick ligation. More extensive analyses using separation-of-function mutants are underway to determine which protein-protein interactions of XRCC1 are biologically critical. Future studies will include the use of animal models to define the relationship of oxidative DNA damage repair to aging and age-related disease.